General strategy Edit
The Saarath fight happens in two separate encounters. If Travel Light is not active, there is a resupply cache in between.
In the first encounter, Saarath will be surrounded by other Qunari, but will not summon his own adds. He only needs to be taken to half health in this encounter. The second encounter is significantly harder. He will lay down more AoE and summon demons periodically. Save tonics and grenades for this fight.
First encounter Edit
Saarath has an AoE, a leap attack, and a barrier that works a bit like Aegis of the Rift. When his barrier is up, projectiles will bounce off of it, so get in close with melee characters or Anchor Blast to deal damage to him.
The other Qunari in the area are likely to be more of a problem in this fight. You can deal with large groups of them by luring them to gaatlok barrels and hitting Anchor Blast to detonate the barrels. The invulnerability will keep you safe from the explosion, but it can instakill some of the Qunari, even on Nightmare difficulty.
You can also use Mark of the Rift if you have one or more of the focus perks from the war table. Anchor Meltdown only triggers when your focus reaches the maximum. If you have 2-3 bars of focus unlocked, you can accumulate enough focus to use normal focus abilities. This can be useful for mopping up larger clusters at range.
Second encounter Edit
The key moment in this fight is in the first couple of seconds, when Saarath leaps down and lands in a crouching position. At this time, you will have a short window to deal as much spike damage as possible before he starts laying down AoE and summoning adds. Use this time to spend your hardest-hitting grenades, tonics, and melee abilities.
If the Inquisitor has one or more of the focus perks from the war table, allow your focus to fill one or two bars before moving forward from the supply cache. Then immediately spend it on Mark of the Rift, Thousand Cuts, or Hail of Arrows. If you have a fully-upgraded Pitch Grenade or Mighty Offense Tonic, now is the time to use those. Also apply Mark of Death if your party contains an assassin.
After the initial grace period, Saarath will begin leaping and laying down AoE as he did in the previous fight. In this encounter, he has more AoE spells at his disposal, and he also starts summoning demons. Demons are most easily cleaned up with Mark of the Rift (if you have enough maximum focus to use it safely) or Confusion Grenade. Be on the lookout for AoE eruptions from the ground, as these deal a significant amount of damage.
When Saarath reaches 0 health, you'll need to use Anchor Blast to finish him.